1. Field of the Invention
This invention relates to a semiconductor device, particularly, an MOS type IC and also to a fabrication process thereof.
2. Description of the Related Art
A semiconductor device generally has a regularly arranged area with a markedly high density and a randomly arranged area with a relatively wide pattern width and distance. A semiconductor memory can be given as one typical example of it. The regularly arranged memory cell region of such a semiconductor memory tends to contain a conductor layer which does not exist in the other regions, leading to a large unevenness on the wafer. This unevenness causes various disturbances in the light-exposure etching step which is indispensable for the fabrication of a semiconductor device, thereby forming a barrier to the miniaturization promotion.
One conventional example of an MOS memory will hereinafter be described with reference to the drawings.
FIGS. 2(a) to (f) illustrate an MOSDRAM formed on a P-type substrate 201. They indicate the fabrication process of a transistor structure until its completion with attention being paid to an N-channel type MOSFET in a memory cell region and an N-channel type MOSFET in a peripheral circuit region. There are products having a P-channel type MOSFET or a bipolar device in the peripheral circuit region but here, such products are omitted for the sake of clear technical explanation. Incidentally, it is easy to incorporate such devices on the same semiconductor device. In Japanese Patent Application Laid-Open No. 259400/1993, specifically described is a process for the fabrication of a DRAM having a CMOS circuit in the peripheral circuit region.
On the P-type semiconductor substrate 201, a thick silicon oxide film is selectively formed as a field insulation layer 202 in an element isolation region by the LOCOS method using a silicon nitride film as an oxidation resistant layer. An appropriate treatment is then given to the active region, whereby a gate insulation layer 203 is formed, for example, to a thickness of 100 xc3x85. All over the surface, a polycrystalline silicon layer 204 which is to be a gate electrode is allowed to grow to a thickness of 2000 xc3x85 by the CVD method [FIG. 2(a)]. At this time, a channel stopper highly-concentrated P+ region, which is not illustrated, is formed in advance directly beneath the field insulation layer 202.
A photoresist 205 is then formed by the light exposure method, whereby a gate electrode 204a is formed. At this time, it is the common practice to carry out simultaneous formation of an N-channel type transistor constituting a memory cell and an N-channel type transistor in the peripheral circuit region [FIG. 2(b)], which is presumed to be conducted in order to avoid an increase in the trend control parameters in the fabrication site. Subsequent to the removal of the photoresist 205, about 2xc3x971013 cmxe2x88x922 of phosphorus is introduced into the substrate 201 in a self-alignment manner by the ion implantation method with the gate electrode 204a and field insulation layer 202 as masks, whereby an nxe2x88x92 (lightly doped) impurity diffusion layer 206 is formed [FIG. 2(c)].
All over the surface, a silicon oxide film of about 1500 xc3x85, for example, is allowed to grow by the CVD method as an insulation layer 207 for the formation of side walls [FIG. 2(d)].
The insulation layer 207 for the formation of side walls is then etched using an anisotropic etching technique, whereby silicon oxide side walls 207 are formed on both sides of each gate electrode as shown in FIG. 2(e).
The memory cell transistor is then covered with a photoresist 209 by the light-exposure method known to date and about 3xc3x971015 cmxe2x88x922of As is introduced into the source and drain regions of the N-channel MOSFET in the peripheral circuit region, whereby an n+ impurity diffusion layer 208 is formed [FIG. 2(f)].
In the above-described manner, an N-channel type MOSFET constituting a memory cell and an N-channel type MOSFET constituting a peripheral circuit are formed. The MOSFET of the peripheral circuit is formed as a so-called LDD transistor having side walls of an oxide film, while that of the memory cell is formed as a single-drain type MOSFET constituted by an nxe2x88x92 impurity diffusion layer.
Through a memory-cell-structure formation step subsequent to the above steps, a memory cell portion having the structure as shown in FIG. 3 is completed. In FIG. 3, indicated at the numerals 301, 302, 304, 306, 307 and 310 are a P-type semiconductor substrate, a field insulation layer, a gate electrode (word line), an nxe2x88x92 impurity diffusion layer, a side wall and an n+ impurity diffusion layer, respectively.
At the opening portions formed on the intrastratum insulation layer on the nxe2x88x92 source and drain regions of the memory cell transistor formed in the step as shown in FIG. 2(f), polycrystalline silicon plugs 311 and 313 are formed and one of them is connected with a tungsten silicide interconnection 312 which is to be a bit line and the other one is connected with a polycrystalline silicon electrode 314 which is to be one electrode of a memory cell capacitor.
In addition, on the surface of the polycrystalline silicon electrode 314, a capacitive insulation layer 315 composed of a silicon oxide film and a silicon nitride film is formed, over which a capacitive polycrystalline silicon electrode 316 is formed as the other electrode of the memory cell capacitor, whereby a memory cell is completed. As needed, an intrastratum insulation layer, contact opening, metal interconnection layer are formed in this order by the method known to date, followed by the formation of a passivation layer, whereby the final structure is completed.
FIG. 4 is an equivalent circuit corresponding to the structure illustrated in FIG. 3. The N-channel type MOSFET constituting a memory cell is formed as a single drain type MOSFET constituted by an n impurity diffusion layer presumably because of the following three reasons: (1) to avoid the influence of crystal defects appearing as a result of the high-concentration ion implantation, (2) to avoid an increase in the leakage current occurring in the region where the heavily-doped impurity diffusion layer is in contact with the channel stopper impurity diffusion layer, and (3) to avoid an increase in the leakage current caused by a punch through between contiguous cells. It becomes very important to satisfy the above three points with the progress of the miniaturization.
Thus, the description has so far been made of, focusing on the process for the fabrication of MOSDRAM. Such a method is however accompanied with the problems because a difference in the density of the patterns between the memory cell region and the peripheral circuit region becomes large and at the same time, the miniaturization in the memory cell region has been drastically accelerated.
In a 64 MDRAM, for example, the gate pitch at the memory cell region reaches about 0.8 xcexcm, while that at the peripheral circuit region remains only 2 to 3 xcexcm. Also in the element isolation region, the memory cell region is formed of maximum density patterns of about 0.3 xcexcm, while the peripheral circuit region is an assembly of rectangles as large as about several tens xcexcm.
In the first place, a serious problem in the light exposure method has been actualized under such situations. Described specifically, it has come to be difficult to conduct size control in the memory cell region and peripheral circuit region, particularly when the size of the memory cell approaches to the resolution limit.
The size control is difficult in both the field formation step and gate formation step and that in the gate formation step is particularly difficult because of the influence of the denseness or sparseness of the underlayer. This is presumed to be caused mainly by the difference in the film thickness of a resist between regions. Relationship among the resist thickness, light exposure amount and finished size becomes complicated by the effects of the standing wave so that the finished size does not simply reflect a change in the other two factors, which leads to increased difficulty in the control.
In the second place, the problem of the conventional device resides in the control of uniformity in dry etching. The influence of the denseness or sparseness of patterns is well known as micro loading effects. The difference in the etching rate between the memory cell region and the peripheral circuit region becomes a problem in various steps and it becomes more apparent owing to an increase in the diameter of the wafer. For example, when in the memory cell region, a gate electrode is formed into an appropriate shape by completely removing the etching remnant, a substrate tends to be damaged at the peripheral portion of the electrode, which has a close relationship with a thinning tendency of a gate insulation film. In addition, when the insulation layer 207 for the formation of side walls as illustrated in FIG. 2(d) is subjected to anisotropic etching, etching sometimes extends to the surface of the substrate, which involves a problem.
This problem is actualized in the case where as described above, the source and drain regions of the MOSFET constituting the memory cell region are formed of nxe2x88x92 impurity diffusion layer, which will be described below with reference to FIG. 5. FIG. 5 is a cross-sectional view illustrating the memory cell portion at the time when the anisotropic etching of the insulation film for side-wall formation is completed. Indicated at the numerals 501, 502, 504, 506 and 507 are a P-type semiconductor substrate, a field insulation layer, gate insulation layer, nxe2x88x92 impurity diffusion layer and a side wall, respectively.
In FIG. 5, the source and drain regions have been dug and the surface portion of the nxe2x88x92 impurity diffusion layer 506 has been removed. As a result, the nxe2x88x92 impurity diffusion layer 506 lacks a relatively heavily-doped portion, leading to an electric situation where the resistance is high and scatters much. There happens a case where current conduction is not provided, though depending on the degree of the scattering. Moreover, owing to the damage of dry etching, defects appear in the nxe2x88x92 impurity diffusion layer 506, which becomes a cause for leakage current and is moreover presumed to bring about incomplete bonding. Such a tendency is considered to be eminent particularly in the end portion contiguous to the LOCOS silicon oxide.
In addition, a decrease in the film thickness of the end portion of the field insulation layer 502 as illustrated in FIG. 5 also leads to a reduction in the isolation capacity of the memory cell portion which is originally critical.
An object of the present invention is to provide a semiconductor device of high reliability. Another object of the present invention is to provide a fabrication process permitting easy control of size and etching and also permitting the widening of the fabrication range.
In the present invention, there is thus provided a semiconductor device which comprises a first MOSFET group of one conductivity type and a second MOSFET group of one conductivity type, each formed on a semiconductor substrate, said first MOSFET group having a side-wall insulation layer on each side of the gate electrode of said first MOSFET group and said second MOSFET group not having a side-wall insulation layer on either side of the gate electrode of said second MOSFET group.
Such a semiconductor device can be fabricated by a process which comprises successively forming a gate insulation layer and a gate conductive layer in an active region on the surface of one conductivity type semiconductor substrate; selectively removing said gate conductive layer to form a gate electrode for the first MOSFET group of one conductivity type while leaving the gate conductive layer portion corresponding to the second MOSFET group of one conductivity type; forming an insulation layer for the formation of side walls on the whole surface and then carrying out etching back to form a side wall on each side of said gate electrode; and selectively removing the remaining gate conductive layer portion to form a gate electrode for the second MOSFET group of one conductivity type.
According to the present invention, a semiconductor device of high reliability can be fabricated by forming a side-wall free MOSFET.
In the fabrication process according to the present invention, pattern formation is carried out separately in a dense region such as memory cell and a sparse region such as peripheral circuit so that it is possible to control the size or etching easily and to widen the fabrication range.